1. Field of the Invention
The present invention relates to an electrochemical machining process for forming surface roughness elements on the recessed back side of a cooling surface of a gas turbine shroud.
2. Description of the Related Art
Gas turbine efficiencies are directly proportional to the temperature of the turbine gases flowing along the hot gas path and driving the turbine blades. Gas turbines typically have operating temperatures on the order of 2700xc2x0 F. To withstand these high temperatures, various parts of the gas turbine require cooling. For example, the shrouds in modern high pressure turbines are typically formed to provide an enhanced cooling surface on the back side recessed portion of the shroud. It will be appreciated that an annular array of shrouds encompasses the hot gas path in the turbine and the surface of each shroud in part defining that hot gas path must be cooled. Typically, a cooling medium such as compressor discharge air or, in more advanced turbines, steam, is directed against the back side cooling surface to maintain the temperature of the shroud within acceptable limits.
In recent times, high pressure turbine shrouds have surface roughness elements cast on their back sides to increase the cooling surface area and improve the overall cooling for the shroud. For example, bumps are often cast in the back side cooling surface of the shroud to provide increased surface area and, hence, increased heat transfer from the shroud wall to the cooling medium as compared with a smooth back side cooling surface. While such cast-in bumps effectively improve the heat transfer coefficient, many older turbines do not have these enhanced cooling surfaces. While it is desirable to provide enhanced heat transfer features on the older shrouds when refurbished or returned from the field for repair, casting is not an option for providing the rough surface elements on existing shroud surfaces.
Recently, an improved electrochemical technique has been developed to provide improved heat transfer characteristics to parts requiring cooling surfaces. The technique is known as STEM (shaped tube electrochemical machining). Aspects of the STEM technique have been described in assignee""s co-pending, commonly-assigned applications Ser. Nos. 60/149,616, titled xe2x80x9cA Method and Tool for Electrochemical Machiningxe2x80x9d; 60/149,618, titled xe2x80x9cA Method and Tool for Electrochemical Machiningxe2x80x9d; 60/149,617, titled xe2x80x9cA Method and Tool for Electrochemical Machiningxe2x80x9d; 09/187,663, titled xe2x80x9cA Method and Tool for Electrochemical Machiningxe2x80x9d (now U.S. Pat. No. 6,200,439); 09/187,664, titled xe2x80x9cProcess for Fabricating a Tool Used in Electrochemical Machiningxe2x80x9d (now U.S. Pat. No. 6,303,193) and 60/149,619, titled xe2x80x9cA Method and Tool for Electrochemical Machining,xe2x80x9d the subject matters of which are incorporated herein by reference. As described in those applications, an electrode is provided having an insulating dielectric material or coating applied on the electrode surface in a pattern which, in conjunction with an electrolyte and the application of an electrical current between the electrode and the workpiece displaces, i.e., dissolves, metal from the adjacent portions of the workpiece to form projections and grooves along the workpiece surface. That is, the metallic portions of the workpiece surface directly adjacent the insulated portions of the electrode are not electrochemically removed, while the portions thereof directly adjacent the non-insulated portions of the electrode are electrochemically removed to form the grooves in the surface of the workpiece.
In accordance with a preferred embodiment of the present invention, an electrochemical machining process is provided for forming surface roughness elements, i.e., raised elements, and spaces therebetween along a back side recessed cooling surface of a shroud, in part defining a hot gas path for a gas turbine. By forming these raised surface elements and spaces therebetween, the heat transfer characteristics of the shroud wall are significantly enhanced. To accomplish the foregoing, the present invention provides, in a preferred embodiment, an electrode, generally in the shape of the back side recessed cooling surface of the gas turbine shroud having an insulating dielectric coating along one surface thereof. Portions of the coating on the electrode surface are removed to form an array of electrical insulating portions and non-insulated portions along that surface. Particularly, the insulating and non-insulating portions of the electrode are preferably formed in a patterned array which, when the electrode surface is disposed in general opposition, lie in opposition to intended locations of the raised surface elements and spaces therebetween on the cooling surface of the shroud. That is, the retained insulated portions of the electrode surface will correspond to the locations of the raised surface elements to be formed along the back side recessed cooling surface of the shroud. The non-insulated portions of the electrode will correspond in location to the spaces to be formed between the surface elements along the back side recessed cooling surface of the shroud. By disposing the electrode with the patterned array of electrically insulating and non-insulating portions in opposition to the back side recessed cooling surface of the shroud and circulating an electrolyte between the electrode and the shroud surface, the application of an electric current between the electrode and the shroud surface electrochemically removes material along the shroud surface adjacent non-insulated portions of the electrode to form the spaces between the raised elements on the shroud surface lying opposite the insulated portions of the electrode.
In a preferred embodiment of the present invention, the electrode surface is initially entirely coated with the dielectric insulating material. Part of the coating is then removed, for example, by using a laser ablation method, to form the array which may be a random or patterned array of insulating and non-insulating portions on the electrode. Preferably, a patterned array, for example, rows and columns of the dielectric material are provided on the electrode surface. Moreover, the shape of the insulated portions or the non-insulated portions on the electrode surface determines the shape of the raised surface elements and spaces therebetween. For example, square, rectilinear, oval or circular-shaped insulating materials formed on the electrode, in turn, form correspondingly-shaped raised elements on the cooling surface of the shroud.
The insulating portions of the dielectric material on the electrode surface may approximate 0.001xc3x970.001 inch to 0.005xc3x970.005 inch and the non-insulated spacing may approximate 0.001 to 0.005 inch. With a patterned electrode employing insulating elements of this size processed on the shroud surface to remove 0.001 inch to 0.005 inch of material therefrom and forming the roughness elements, it will be appreciated that the surface area of the cooling surface of the shroud is substantially increased. Using those preferred sizes, the roughness elements on the cooling surface can double the heat transfer surface area. Since the surface heat transfer rate is proportional to the surface area, surface cooling can be significantly enhanced. When these roughened surfaces are used for impingement cooling, the heat transfer can be improved by at least 50%. While the electrochemical machining process hereof may be used on original equipment, it comprises a significant increase in heat transfer characteristics for the older shrouds when serviced or repaired to reduce the metal temperature and enhance the service life of the shroud.
In a preferred embodiment according to the present invention, there is provided a process for forming raised elements and spaces therebetween along a back side recessed cooling surface of a shroud in part defining a hot gas path for a gas turbine, comprising the steps of (a) locating an electrode having electrical insulating material arranged in an array along a surface thereof defining insulated and non-insulated portions of the electrode surface in general opposition to intended locations of the raised elements and spaces therebetween, respectively, on the cooling surface, (b) flowing an electrolyte between said electrode surface and the cooling surface of the shroud and (c) passing an electric current between the electrode and the shroud to form the raised elements and spaces therebetween along the cooling surface of the shroud.